Method for producing bisphenol-A

ABSTRACT

A process of preparing bisphenol A having low residual contents of oxygen and phenol is described. The process involves: (a) melting mixed crystals of bisphenol A and phenol under a nitrogen atmosphere at a temperature of 100° C. to 120° C.; (b) feeding continuously the melt formed in step (a) into the top of a distillation unit under conditions of 120° C. to &lt;160° C. and a pressure of &gt;80 mbar to 200 mbar; (c) feeding contemporaneously with step (b) nitrogen into the bottom of the distillation unit; (d) allowing the melted feed material and the nitrogen feed to contact each other within the distillation unit, thereby forming a concentrated melt material having (i) a phenol content reduced to 10 to 25 wt. %, and (ii) a reduced oxygen content (e.g., an oxygen content of &lt;1 ppm); and (e) optionally passing a stream of nitrogen through the concentrated melt material under conditions of at least atmospheric pressure, 180° C. to 220° C. and a nitrogen partial pressure of 1 to 1.5 bar, thereby removing residual phenol from the concentrated melt material (e.g., resulting in a phenol content of &lt;50 ppm). The melted material having reduced oxygen and phenol contents may be further processed, for example, into pellets.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

The present patent application is a 371 of International Application No.PCT/EP99/09917, filed Dec. 14, 1999, which was published in German asInternational Patent Publication No. WO 00/39060 on Jul. 6, 2000, whichis entitled to the right of priority of German Patent Application No.198 60 144.1, filed Dec. 24, 1998.

FIELD OF THE INVENTION

The present invention relates to a process for the preparation of abisphenol A which is light in colour and has low residual contents ofoxygen and phenol, starting from mixed crystals of bisphenol A andphenol.

BACKGROUND OF THE INVENTION

EP-A 523 931 discloses a process wherein an adduct of bisphenol A andphenol is melted in an atmosphere containing 50 ppm at most, preferably10 ppm at most, of oxygen. The melting is carried out at 115° C. to 180°C., preferably 120° C. to 150° C., at a pressure of 1.0 to 5.0 atm.,preferably 1.0 to 1.9 atm. Phenol is removed from the melt byevaporation. The temperature is 160° C. to 200° C., preferably 170° C.to 185° C.; the pressure is at most 100 torr (133 mbar), preferably 5 to40 torr (7 to 53 mbar). Preferably, the bulk of the phenol is first ofall distilled off in a downflow evaporator at 160° C. to 185° at 20 to80 mbar to a residual content of 1 to 5wt. %, and the residual phenol isthen removed by steam at 20 mbar at most and at 170° C. to 185° C. Theratio of steam:BPA is from 1:50 to 1:5, preferably 1:25 to 1:10. Inorder to effect the separation of the phenol from the BPA as far aspossible with the exclusion of oxygen, it is proposed that all thesurfaces of the apparatus used which are in contact with the product befreed from oxygen by means of an organic solvent, preferably phenol. Thebest bisphenol products obtained from the directions given in EP-A 523931 have colour values (APHA) of 10.

SUMMARY OF THE INVENTION

A process has now been found whereby products having even better colourvalues can be obtained, and which moreover renders unnecessary theelaborate washing step for removing traces of oxygen from the apparatus.

The invention provides a process for the preparation of bisphenol A,wherein a mixed crystallisate of bisphenol A and phenol is melted attemperatures of 100° C. to 120° C., the melt is then rendered inert withnitrogen and fed in continuously at the top of a first distillation unitwherein, at temperatures of 120° C. to <160° C., preferably 120° C. to157° C., and at pressures of >80 to 200 mbar, preferably 85 to 200 mbar,particularly preferably 85 to 120 mbar, the phenol content of the meltis decreased to 10 to 25 wt. %, and at the same time 0.5 to 50 vol. %,preferably 2 to 20 vol. %, of nitrogen, based on the volume ofintroduced melt, is passed in via the bottom of the distillation unit,and the concentrated melt freed from oxygen is subsequently freed fromresidual phenol at temperatures of 180° C. to 220° C. and at 1 to 1.5bar nitrogen partial pressure.

The invention also provides a process for the preparation of meltscontaining 75 to 90 wt. % bisphenol A and 10 to 25 wt. % phenol andhaving an oxygen content of <1 ppm, preferably <100 ppb, particularlypreferably <10 ppb, wherein mixed crystals containing 60 wt. % bisphenolA and 40 wt. % phenol are melted at temperatures of 100° C. to 120° C.,the melt is then rendered inert with nitrogen and fed in continuously atthe top of a distillation unit wherein, at temperatures of 120° C. to<160° C., preferably 120° C. to 157° C., and at pressures of >80 to 200mbar, preferably 85 to 200 mbar, particularly preferably 85 to 120 mbar,the phenol content of the melt is decreased to 10 to 25 wt. %, and atthe same time 0.5 to 50 vol. % of nitrogen, preferably 2 to 20 vol. %,of nitrogen, based on the volume of introduced melt, is passed in viathe bottom of the distillation unit.

DETAILED DESCRIPTION OF THE INVENTION

In the process according to the invention, mixed crystallisates ofbisphenol A and phenol are used as educt, preferably those having abisphenol content of 60 wt. % and a phenol content of 40 wt. %. Suchmixed crystallisates are obtained, for example, during theacid-catalysed reaction of phenol and acetone, which is described, forexample, in U.S. Pat. No. 2,775,620 or EP-A 342 758. As a rule, thepreparation is conducted as a continuous process. The mixedcrystallisate of bisphenol A and phenol obtained as product is separatedoff by filtering the mother liquor, for example, by means of a rotaryfilter. The filter cake can then be washed with phenol in order toremove adhering impurities. The resulting cake of mixed crystals can beused as educt in the process according to the invention.

In a first step, the mixed crystallisate is melted at temperatures of100° C. to 120° C. This is carried out in a nitrogen atmosphere. Becauseof the low reactivity of bisphenol with oxygen at these lowtemperatures, it is not necessary here to ensure that oxygen is totallyexcluded. Oxygen contents of 0.1 to 2 vol. % can therefore be toleratedin the atmosphere during the melting process. It is advisable to collectthe melt in a container rendered inert with nitrogen.

The melt, which may still contain dissolved oxygen, is then fed incontinuously at the top of a distillation unit. It is important that aslarge an exchanging surface as possible be produced in this distillationunit. Accordingly, a packed column, packing column, tray column or adownflow evaporator, film evaporator or an evaporator with forcedcirculation is preferably used for this step.

In the distillation unit, at temperatures of 120° C. to <160° C.,preferably 120° C. to 157° C., and at pressures of >80 to 200 mbar,preferably 85 to 200 mbar, particularly preferably 85 to 120 mbar, thephenol content of the melt is decreased to 10 to 25 wt. %, and at thesame time 0.5 to 50 vol. % of nitrogen, preferably 2 to 20 vol. %, ofnitrogen, based on the volume of introduced melt, is passed in via thebottom of the distillation unit.

It is important in this step that melt and nitrogen be passed through incountercurrent, in order to discharge the dissolved oxygen in the meltas completely as possible. Here the removal of the oxygen is assisted bythe evaporation of phenol and by the lowered pressure. Under the givenconditions, the stream of nitrogen assists the removal of oxygen withoutlowering the partial pressure of the phenol. In the first distillationunit, dissolved oxygen is removed from the melt under mild conditions,leaving a residual content of <1 ppm. Melts having oxygen contents of<100 ppb and even <10 ppb can be obtained by optimal selection of theconditions. Moreover, no losses of bisphenol A occur under theconditions of pressure and temperature established in this step; suchlosses would necessitate an additional processing step in whichbisphenol A is recovered from the vapour phase.

In a further step, the melt thus obtained can then be freed fromresidual phenol at temperatures of 180° C. to 220° C., preferably 185°C. to 195° C., and nitrogen partial pressures of 1 to 1.5 bar,preferably 1 to 1.1 bar. In the course of this, a stream of nitrogen ispassed through the melt in order to assist the removal of the phenol(“stripping”). Here the proportion of gas to melt is preferably 10 to1000 m³ nitrogen per t of melt. The units employed for the removal ofphenol are familiar to the person skilled in the art; for example, apacked column operated by flooding can be used for this.

The removal of phenol at atmospheric pressure enables the melt to berendered inert better than is possible by the desorption under vacuumconditions as described in EP-A 523 931 and has the advantage, moreover,that no oxygen can flow into the apparatus in the event of leakages.Furthermore, the apparatus is rendered inert by the stream of nitrogenand any oxygen which may be adsorbed onto the surfaces in contact withthe product is swept out, which renders washing of these surfaces withorganic solvent superfluous. The bisphenol A thus obtained has aresidual phenol content of 50 ppm at most and shows colour values(measured in accordance with ASTM D 1686) of less than 10.

The melt can subsequently be processed directly into pellets or used forthe preparation of sodium bisphenolate solutions for polycarbonateproduction. Polycarbonates produced from bisphenol A obtained by theprocess according to the invention have a lower Yellowness Index (Y.I.)than that of the products of prior art. The bisphenol A obtained by theprocess according to the invention can also be used with advantage forthe preparation of epoxy resins.

EXAMPLES Example 1

A melt which contained 60 wt. % bisphenol A and 40 wt. % phenol, and inwhich oxygen was still dissolved, was fed in continuously at the top ofa packed column in a quantity of 1 t/h. The temperature at the bottom ofthe column was 145° C.; the pressure at the top of the column was 100mbar. 50 l of nitrogen per hour was passed continuously to the downflowevaporator of the column in countercurrent to the flow of product. In asubsequent desorption step, at a melting temperature of 190° C., themixture discharged from the column (75 wt. % bisphenol A, 25 wt. %phenol) was freed from phenol by means of 225 m³ of nitrogen per hour.The phenol contained in the circulating nitrogen was condensed. 750 l ofthe recirculated nitrogen was exchanged per hour. The oxygen content ofthe circulating nitrogen was 0.4 ppm. A BPA melt was obtained which waslow in phenol (50 ppm) and had a colour value of 8 Hazen (measured inaccordance with ASTM D 1686).

Comparison Example 1

The procedure was carried out as in Example 1, but the continuous supplyof nitrogen to the downflow evaporator was dispensed with. The oxygencontent of the circulating nitrogen was 5 ppm. A BPA melt was obtainedwhich was low in phenol (50 ppm) and had a colour value of 12 Hazen.

Comparison Example 2

A melt which contained 60 wt. % bisphenol A and 40 wt. % phenol, and inwhich oxygen was still dissolved, was passed continuously in a quantityof 1 t/h directly into the desorption step and, at a melting temperatureof 190° C., was freed from phenol by means of 225 m³ of nitrogen perhour. The phenol contained in the circulating nitrogen was condensed.750 l of the recirculated nitrogen was exchanged per hour. The oxygencontent of the circulating nitrogen was 5 ppm. A BPA melt was obtainedwhich had a phenol content of 250 ppm and had a colour value of 14Hazen.

Comparison Example 3

The procedure was carried out as in Comparison Example 2, but the phenoldesorption was operated at a melting temperature of 200° C. in order tolower the phenol content further. A BPA melt was obtained which was lowin phenol (50 ppm) and had a colour value of 18 Hazen.

What is claimed is:
 1. A process of preparing bisphenol A having lowresidual contents of oxygen and phenol comprising: (a) melting mixedcrystals of bisphenol A and phenol under a nitrogen atmosphere at atemperature of 100° C. to 120° C.; (b) feeding continuously the meltformed in step (a) into the top of a distillation unit under conditionsof 120° C. to less than 160° C. and a pressure of greater than 80 mbarto 200 mbar; (c) feeding contemporaneously with step (b) 0.5 to 50 vol.% of nitrogen, based on the volume of introduced melt, into the bottomof the distillation unit; (d) allowing the melted feed material and thenitrogen feed to contact each other within the distillation unit,thereby forming a concentrated melt material having, (i) a phenolcontent reduced to 10 to 25 wt. %, and (ii) a reduced oxygen content;and (e) removing the concentrated melt material of step (d) from thebottom of the distillation unit and passing a stream of nitrogen throughthe removed concentrated melt material under conditions of at leastatmospheric pressure, 180° C. to 220° C. and a nitrogen partial pressureof 1 to 1.5 bar, thereby further reducing the residual phenol content ofthe concentrated melt material.
 2. The process of claim 1 wherein themixed crystals of bisphenol A and phenol melted in step (a) contain 60percent by weight of bisphenol A and 40 weight percent of phenol, basedon the total weight of the mixed crystals.
 3. The process of claim 1wherein the oxygen content (ii) of the concentrated melt material ofstep (d) is less than 1 ppm.
 4. The process of claim 1 wherein thephenol content of the concentrated melt material of step (e) is lessthan 50 ppm.
 5. The process of claim 1 wherein the proportion ofnitrogen gas to concentrated melt material in step (e) is 10 to 1000 m³of nitrogen per ton of concentrated melt material.
 6. The process ofclaim 1 wherein the concentrated melt material of step (e) is furtherprocessed into the form of pellets.
 7. A process of preparing bisphenolA having low residual contents of oxygen and phenol comprising: (a)melting mixed crystals comprising 60 wt. % of bisphenol A and 40 wt. %of phenol under a nitrogen atmosphere at a temperature of 100° C. to120° C.; (b) feeding continuously the melt formed of step (a) into thetop of a distillation unit under conditions of 120° C. to less than 160°C. and a pressure of greater than 80 mbar to 200 mbar; (c) feedingcontemporaneously with step (b) 0.5 to 50 vol. % of nitrogen, based onthe volume of introduced melt, into the bottom of the distillation unit;(d) allowing the melted feed material and the nitrogen feed to contacteach other within the distillation unit, thereby forming a concentratedmelt material having, (i) a phenol content of 10 to 25 wt. %, (ii) abisphenol A content of 75 to 90 wt. %, and (iii) an oxygen content ofless than 1 ppm; and (e) removing the concentrated melt material of step(d) from the bottom of the distillation unit.
 8. The process of claim 7wherein the concentrated melt material of step (e) is further processedinto the form of pellets.